1. Field of the Invention
The present invention relates to an internal combustion engine control apparatus and method, which computes the quantity of air to be charged into each cylinder of an internal combustion engine provided with a variable valve mechanism for changing at least one of valve timing and valve lift characteristics.
2. Related Art
Recently, internal combustion engines for vehicles are equipped with a variable valve timing mechanism and a variable valve lift mechanism (hereinafter collectively referred to as the "variable valve mechanism") for the purpose of improving engine output, reducing fuel consumption, and reducing exhaust emission. In the internal combustion engine having the variable valve mechanism, an air flow meter (or an intake air pressure sensor) mounted in an intake pipe is adopted for detecting the quantity of intake air and calculating the quantity of air (cylinder-charged air quantity) to be charged into the cylinder in accordance with the detected intake air quantity (or the intake pipe air pressure).
If the valve timing or valve lift characteristic of the intake/exhaust valve varies, the amount of overlap of the intake/exhaust valve opening period varies and the amount of EGR (the ratio of residual exhaust gases) in the internal combustion engine varies, thereby the quantity of air to be charged into the cylinder varies. There is, however, a time delay until the detection, by the air flow meter (or the intake air pressure sensor), of the change in the quantity of air to be charged into the cylinder. This is because a surge tank of a relatively large volume mounted in the intake air passage between the air flow meter (or the intake air pressure sensor) and the internal combustion engine delays pressure propagation from the internal combustion engine to the air flow meter (or the intake air pressure sensor). Therefore there arises a problem that the quantity of air to be charged into the cylinder can not accurately be computed by the use of the detected value from the air flow meter (or the intake air pressure sensor).
For computing the quantity of air to be charged into the cylinder from detected values (hereinafter called the "detected load value") of the quantity of intake air or the intake air pressure, there has been proposed by JP-B2-6-21592 a method for computing the quantity of air to be charged into the cylinder by the first-order delay processing of the detected load value. This method aims at correcting the characteristics that, in the system provided with a throttle valve in the intake air passage, the detected load value becomes excessively large or excessively small with respect to the actual quantity of air to be charged into the cylinder, when the throttle valve opens or closes rapidly.
In the system for computing the quantity of air to be charged into the cylinder through the first-order processing of the detected load value as stated above, when the variable valve mechanism is adopted, for instance when the variable valve mechanism has varied to increase the quantity of air to be charged into the cylinder, the detected load value is further subjected to the first-order delay processing notwithstanding the detected load value is smaller than the actual quantity of air to be charged into the cylinder. It is, therefore, possible that the computed quantity of air to be charged into the cylinder will differ largely from the actual quantity of air to be charged into the cylinder.